1. Technical Field
The present invention relates to an inductance variable device in which inductance can be varied.
2. Background Art
An inductor device in which inductance can be varied by electrical means is used for such as a resonance circuit in which a resonance frequency can be varied over a large frequency range. In the description below, an element having an inductance such as a coil is referred to as an “inductor”, and a device comprising inductors is referred to as an “inductor device”.
An example of conventional inductor devices is disclosed in Japanese Patent Laid-open Publication No. 2002-151953. This inductor device will be described with reference to FIGS. 8 to 10.
An inductor device 40 shown in FIG. 8 includes a square frame-shaped main inductor 41 formed of a conductor on an insulation substrate (not shown), and a sub inductor 42 formed outside the main inductor 41 and having a square frame shape larger than that of the main inductor 41. A part of the frame of the main inductor 41 is cut so that terminals 41a and 41b are led to the outside. In addition, a part of the frame of the sub inductor 42 is cut so that terminals 42aand 42b are formed. A first switch 44 serving as a semiconductor switch is connected to the terminals 42a and 42b. In addition, a second switch 46 is connected between the terminal 42b and a circuit ground G.
When an alternate current is applied to the main inductor 41 and the first switch 44 is closed to short-circuit the terminals 42a and 42b, inductance of the main inductor 41 is varied by magnetic coupling. When the first switch 44 is closed, the second switch 46 is opened. When the first switch 44 is opened, the second switch 46 is closed and the sub inductor 42 is grounded.
According to another conventional inductor device 50 shown in FIG. 9, inside a square frame-shaped main inductor 51, a square frame-shaped sub inductor 52 which is smaller than the main inductor 51 is provided. A part of the frame of the main inductor 51 is cut so that terminals 51a and 51b are led to the outside. In addition, a part of the frame of the sub inductor 52 is cut so that terminals 52a and 52b are provided, and a first switch 53 is connected between the terminals 52a and 52b. In addition, a second switch 54 is connected between the terminal 52b and a circuit ground G. When an alternate current is applied to the main inductor 51 and the first switch 53 is closed to short-circuit the terminals 52a and 52b, inductance of the main inductor 51 is varied by magnetic coupling. When the first switch 53 is closed, the second switch 54 is opened. When the first switch 53 is opened, the second switch 54 is closed and the sub inductor 52 is grounded.
According to still another inductor device 60 shown in FIG. 10, a spiral shaped sub inductor 62 is formed so as to overlap with a spiral main inductor 61. A first switch 63 is connected between terminals 62a and 62b of the sub inductor 62. In addition, a second switch 64 is connected to the terminal 62b and a circuit ground. When an alternate current is applied to the main inductor 61 and the first switch 63 is closed to short-circuit the terminals 62a and 62b, inductance of the main inductor 61 is varied by magnetic coupling. When the first switch 63 is closed, the second switch 64 is opened. When the first switch 63 is opened, the second switch 64 is closed and the sub inductor is grounded. According to the constitutions shown in FIGS. 8 to 10, the inductance of each of the inductor devices 40, 50 and 60 can be varied by increasing or decreasing the number of spirals of the main inductor.
In addition, Japanese Patent Laid-open Publication No. 2002-9544 discloses a voltage control oscillator including an inductor for the oscillator and a control inductor. According to this voltage control oscillator, a current to be applied to the control inductor is set to a predetermined value and a predetermined mutual inductance is generated in the inductor for the oscillator.